1. Field of the Invention
The present invention relates to a focus detecting device for use with cameras, which is adapted to measure object lights having passed through our objective lens (i.e., picture taking lens) for detecting the focus condition of an object image formed on a film exposure plane.
2. Description of Prior Art
There have been proposed first and, second types of focus detecting devices for detecting a correlation between two object images, which are formed by first and second parts of object lights having passed through an objective lens at its first and second portions distant from the optical axis, respectively. A principle construction of the optical system used in the first types of focus detecting devices is shown in FIG. 1. At a position equivalent to a predetermined focal plane of an objective lens 2 (i.e., a film exposure plane) there is disposed a condensor lens 4 and behind this condensor lens are provided a pair of image forming lenses 6 and 8 and a pair of line sensors 10 and 12. Line sensors 10 and 12 are disposed on image forming planes of image forming lenses 6 and 8 and each of them is composed of a CCD (change coupled device). In a front focus condition where an object image to be the subject of the focus detection is formed in front of the predetermined focal plane of the objective lens, two images 14 and 16 formed by image forming lenses 6 and 8 on line sensors 12 and 14 respectively are near the optical axis 18 of the objective lens. In contrast, two images 14 and 16 are remote from the optical axis in a rear focus condition where the object image is formed behind the predetermined focal plane of the objective lens. In an in-focus condition where the object image is formed on the predetermined focal plane, the distance between the corresponding points of two images 14 and 16 becomes a specific length determined by the construction of the optical system. Accordingly, if the patterns of the light distributions on line sensors 10 and 12 are converted into electrical signals respectively, the focus condition can be found out by comparing these electrical signals to detect the positional relationship between the two images. This type of focus detecting devices is disclosed, for example, in Japanese utility model laid-open publication No. Sho. 55-157220, Japanese patent laid-open publication Nos. Sho. 52-95221, Sho. 55-45031, Sho. 55-98709, Sho. 55-98710 and Sho. 55-146423 and U.S. Pat. No. 3,875,401.
In U.S. Pat. No. 4,333,307 which disclose the second type of focus detecting device for detecting a correlation between two object images, and a method to detect both the amount and direction of defocus of an object image at a single time is disclosed. In this patent a number of detector pairs each consisting of first and second small detectors are arranged behind a small lenslet which is in turn arranged on a predetermined focal plane of an objective lens and any correlation between two object images, one being formed on the first detectors of the pairs and the other on the second detectors of the pairs, is detected by comparisons between the outputs of the first detectors and those of the second detectors. The comparisons are carried out by shifting the outputs of the second detectors one by one and by changing the number of outputs of the first and second detectors to be compared with one another. The reason for the change of the number of the outputs is that, with the outputs of the second detectors shifted, some of the outputs of the first detectors will have no counterpart to be compared therewith in the outputs of the second detectors. If a coincidence is found between the outputs of the first detectors and those of the second detectors by a comparison at a certain shift position, then the shift position is indicative of the amount of defocus and the direction of the shift is indicative of the direction of defocus. This method disclosed in U.S. Pat. No. 4,333,307 will be similarly applicable to the first type focus detecting devices as mentioned above with reference to FIG. 1. However, there is a problem in the method, in that the results of the comparisons between the outputs of the first detectors and those of the second detectors cannot be equally evaluated because the number of outputs of the first and second detectors is different for each comparison. Thus, some conditions on the side of an object will cause failure in finding out a comparison result which represents in a coincidence between the outputs of the first detectors and those of the second detectors.
By the way, there are still problems in order to put the first type of focus detecting type into practical use. One of the problems is that it is difficult to assemble the optical system exactly in the same manner as it is designed, so that the distance between the corresponding points of two images 14 and 16 becomes different for each individual product even in the same focus condition. This makes it hard to manufacture this type of focus detecting devices with a required operational accuracy. Additionally, the magnification of the two images formed on the line sensors varies with the focus condition of the object image and this also undesirably affects the distance between the two images to be detected. Thus, accurate detection of the amount of defocus cannot be expected without correction of the detected distance by the magnification data of the two images. Moreover, in the case where the line sensors 12 and 14 consist of CCDs, it becomes necessary to make electric charges stored therein completely empty before actually starting a focus detection operation. To this end, when a focus detection start switch is closed, transfer pulses are sent to the CCDs and a transfer operation to discharge the electric charges stored in the CCDs is repeatedly carried out several times in accordance with the transfer pulses. However, since transfer pulses substantially equal to the number of the picture elements of the CCDs are necessary for each time of a transfer operation, it takes a considerably long time to complete the several transfer operations and this results in significant retardation of the start of the actual focus detection. Preferably this problem should also be solved in order to put the first type of focus detecting device into practical use.